We report results on the photodissociation dynamics of the tert-butyl radical, C 4 H 9 . A beam of internally cold radicals is generated by supersonic jet flash pyrolysis of azo-t-butane. Upon optical excitation, the radical dissociates into 2-methylpropene (iso-butene) and a hydrogen atom that is detected by time-resolved photoionization. By varying the excitation wavelength between 355 and 266 nm, unimolecular dissociation rates are obtained as a function of excitation energy. Whereas at low excitation energies, the rates are on the order of 10 8 s -1 , in agreement with simple RRKM calculations. However, the reaction slows down at excitation wavelengths shorter than 329 nm. The data are discussed in light of previous results on the ethyl radical.